Forums new post form should reset state when user cancels

TNL-148
You're right, I withdraw my suggestions for the 2nd sentence until I can see a screen shot of what this looks like in any browser. I can't think of anywhere that we warn users that work will be deleted in the LMS (and we throw their work away a lot if they step out of line), so I thought this might be like a cancel action in Studio, but apparently not.